The present invention relates to a silver plating solution and silver plating process in which immersion deposition of silver on a substrate having a copper or copper alloy surface is effectively prevented.
Silver plating has heretofore been conducted by using a cyanide plating bath containing a large amount of a cyanide, but since Lerner proposed a low-cyanide silver plating bath in 1977 (U.S. Pat. No. 4,024,031), with development of the high-speed jet plating process, low-cyanide baths have been mainly utilized in practice for plating lead frames for semiconductors. In this field, in view of the problems on the use of saving of noble metals, the electro-migration of silver, and the soldering characteristics and the like, there has been developed a selective plating technique in which plating is effected only in a functionally required area such as a bonding area. When a silver plating solution comes in contact with copper or copper alloy, because of the difference of the ionization tendency between copper and silver, as is well-known, silver is readily immersion-deposited on copper. This phenomenon is one of the causes of the poor adhesion of the silver plating. In order to avoid occurrence of this undesirable phenomenon, it is indispensable to perform strike plating in advance. However, when the recently developed low-cyanide silver plating bath is employed, immersion deposition of silver is considerably moderated, and a certain adhesion is obtained even if silver plating is carried out without performing strike plating. However, in the selective plating process, in which frames are sealed with two masks and then a mask-plate, immersion deposition of silver is caused on the plated portion and the unplated portion for which plating is unnecessary, because of splashing of the plating solution at the transporting step and contamination of the mask surfaces with the silver plating solution, resulting in an uneven appearance or blistering. As means for solving this problem, Nobel et al proposed a process in which immersion deposition of silver is moderated by incorporating a mercaptan compound into a silver plating bath (U.S. Pat. No. 4,247,372). However, this process is still insufficient in some points. For example, in the case of bright plating, detrimental influences are produced in a low-current portion, and even though immersion deposition of silver is moderated, the deposition of silver is in such an amount as often causing blistering. Moreover, in a cyanide-containing bath, if the cyanogen concentration is higher than 2 g/l, the anti-immersion effect by the mercaptan compound is reduced and the decomposition speed of the compound is increased. In case of semi-bright or matte plating, if the thiolactic acid concentration is higher than 0.03 ml/l, the plating appearance is drastically degraded. In order to mass-produce selectively silver-plated articles having a high quality, therefore, it is necessary to eliminate the foregoing defects and develop an additive to a silver plating solution, which has an enhanced effect of preventing immersion deposition of silver and can also be used for a treating solution for performing an anti-immersion treatment before the silver plating step.
As discussed above, the background for development of an anti-immersion agent has been produced by the conversion of the cyanide bath to the low-cyanide bath in which the anti-immersion effect is relatively easy to obtain. However, the silver plating solution contains silver as cyanide salt so cyanogen is accumulated in the plating bath in accordance with the plating operation. Accordingly, development is desired of a new additive capable of exerting an anti-immersion effect even in the presence of free cyanogen.
Alphatic mercaptans have heretofore been used as the anti-immersion agent. These mercaptans, however, have a bad smell and their anti-immersion effect is insufficient. Since an anti-immersion agent is incorporated in a silver plating bath, it is required that the anti-immersion effect should not be degraded even if cyanogen is accumulated in the plating bath. However, the above-mentioned mercaptan type anti-immersion agents do not show any effect in the conventional cyanide silver plating baths. Namely, if cyanogen is present in the plating bath, the effect of the mercaptan type additive is drastically reduced, and if the cyanogen concentration is higher than 2 g/l, blistering is often caused. This problem may be solved by maintaining the anti-immersion effect at a high level by controlling the cyanogen concentration to a low level. For example, a considerable effect can be attained in a process in which silver cyanide (AgCN) packed in a bag is suspended as the silver source in the plating bath, or silver cyanide is charged in a filter pump as carbon in case of the carbon treatment, and cyanogen formed in the plating bath is absorbed by reacting it with silver cyanide as indicated by the following reaction formula: EQU AgCN+CN.sup.- .fwdarw.Ag(CN).sub.2.sup.-
Generally speaking, however, from a practical viewpoint, it is desirable to develop a more superior anti-immersion agent, the effect of which will not be degraded even if the cyanogen concentration in the plating bath is as high as about 15 g/l. In the plating processes now adopted in the art, the anti-immersion agent also acts as a brightening agent, and the concentration is set so that both the functions can be exerted. However, the concentration suitable for the brightening agent is 1/50 or less of the concentration required for the anti-immersion agent. Even in case of bright plating, the defect due to addition of too large an amount of the anti-immersion agent is prominently manifested in a low current density portion where the current density is lower than 70 A/dm.sup.2 in the jet plating process. In this case, however, a good bright plating is obtained in a region where the current density is 80 to 150 A/dm.sup.2, and the above defect, therefore, is seemingly not so serious. However, in the low density portion of the plating, for example, defects are readily caused in the end face or side face of the mask or the back surface of the plated surface due, to insufficient plating, such as peeling and blistering, and these defects are very serious in practical operation. In case of matte silver plating, if the anti-immersion agent is incorporated into the plating bath in an amount sufficient to exert an anti-immersion effect, the range of the current density providing a good plating is extremely narrowed or not present at all, though this range differs to some extent according to the kind of the plating bath. As a means for avoiding this disadvantage, there has been adopted a method in which the anti-immersion treatment is carried out by pre-dipping, and the pre-dipped substrate is washed with water so as to control the amount of the anti-immersion agent to be dragged into the silver plating bath. However, even if this method is adopted, yelowing occurs on the plated surface within several days without fail, resulting in reduction of the current density used. Accordingly, it is impossible to use the plating bath for a long time. In view of the foregoing, it is desirable to develop an anti-immersion agent which exerts only the anti-immersion function when incorporated in a plating bath and which is inactive to the plating reaction.
As is apparent from the foregoing description, anti-immersion agents now available are insufficient in the following points. Namely, the anti-immersion effect is low, the effect is degraded in the presence of cyanogen, and, in the case of matte plating, detrimental influences are given to the plated surfaces to such an extent that practical application is not permissible.